Let's take a completely generic Guard character with no powerups or items other than the Mage Fist and a 12 point mana pool. For use of discussion, let's say he's 5th level and is facing an equal level monster from which he can soak two hits safely.

Without the Mage Fist, he can strike twice, inflicting 25x2=40 damage, plus he can cast two fireballs, adding in another 20X2=40, for a grand total of 80 damage.

So that's a tie, except that you lose the flexibility normally offered by Fireball and you had to use your locker slot to do it. Worse, if you have made any improvements to your attack strength or number of fireballs, either through class abilities, powerups, boons, or items, then the Mage Fist actually reduces your overall damage.

What about resistances? Let's take our 5th level Guard and grant him 50% resistances. How does that work?With Mage Fist:2 Fireballs for 40x2= 80 damage2 Attacks for 25x2 = 50 damage130 damage total

So resistances also hurt the Mage Fist. That makes sense, since anything that improves your melee efficiency is going work counter to the Mage Fist. So is there any way to use this thing at all?

Well, as others have pointed out, you can combo it with slow effects to front load an extra fireball's worth of damage. To pull this off, you need three components (Mage Fist + Fireball + WEYTWUT/Earthmother). It's okay, and stacks with other free hit effects like first strike, dodge, and death protection, but I'm not sure that I'm sold on it.

What about low damage characters? Can they make more general use of it? Let's send in a 5th level Monk against the same nameless monster that we've been throwing our Guard at.With Mage Fist2 Fireballs for 40x2 = 80 damage2 Attacks for 15x2 = 30 damage110 damage total

Not even then. Let's take a Monk with 18 manaMage Fist3 Fireballs 40x3 = 1201 Attack 15x1= 15135 Total

No Mage Fist3 Fireballs 20x3 = 604 Attacks 15x4 = 60120 Total

That's a very marginal gain that will get smaller and smaller as you gain attack boosts. Plus you lose the ability to get unreciprocated spell attacks, which is a pretty big loss. Right now, I'd say that taking nothing is very competitive with prepping the Mage Fist.